Enums

Numeric enums, string enums, const enums, and when to use union types instead.

Overview

Enums (enumerations) define a set of named constants. Instead of scattering magic strings or numbers throughout your code, you give them meaningful names grouped in one place. TypeScript has two main enum flavours: - **Numeric enums** members are automatically assigned integers (0, 1, 2, …) or manually assigned numbers - **String enums** members have explicit string values; safer and more readable in logs and APIs **`const enum`** is optimised: the compiler inlines the values directly (no runtime object is generated), resulting in smaller output. However, many TypeScript teams now prefer **union types with literal strings** over enums: `type Status = "active" | "inactive" | "pending"`. They are simpler (no special syntax), work seamlessly with JSON, and are easier to understand for newcomers. Both patterns are valid use whichever your team prefers.

Key Points

•Numeric enum: `enum Direction { North, South, East, West }` starts at 0 by default
•String enum: `enum Color { Red = "RED", Green = "GREEN" }` explicit string values
•`const enum` inlines values at compile time no runtime object generated
•Enum members are accessed like: `Direction.North`, `Color.Red`
•Numeric enums allow reverse mapping: `Direction[0]` → `"North"`; string enums do not
•Union literal alternative: `type Status = "open" | "closed" | "pending"` often preferred
•Use string enums (or union literals) when values appear in logs, APIs, or persisted data

Examples

Numeric and string enums

// Numeric enum (auto-increments from 0)
enum Direction {
  North,   // 0
  South,   // 1
  East,    // 2
  West,    // 3
}

console.log(Direction.North);    // 0
console.log(Direction[0]);       // "North" (reverse mapping)

// Custom start value
enum Priority {
  Low = 1,
  Medium,   // 2
  High,     // 3
  Critical = 100,
}

// String enum
enum Status {
  Active   = "ACTIVE",
  Inactive = "INACTIVE",
  Pending  = "PENDING",
}

console.log(Status.Active);     // "ACTIVE"
// String enums have NO reverse mapping
console.log(Status["ACTIVE"]);  // undefined

Using enums in functions

enum LogLevel {
  Debug = 0,
  Info  = 1,
  Warn  = 2,
  Error = 3,
}

function log(level: LogLevel, message: string): void {
  const prefix = LogLevel[level];    // reverse mapping
  console.log(`[${prefix}] ${message}`);
}

log(LogLevel.Info, "Server started");   // [Info] Server started
log(LogLevel.Error, "DB connection failed");  // [Error] DB connection failed

// TypeScript prevents passing arbitrary numbers to enum params
// log(99, "test");   // Works at runtime but enum doesn't cover 99

const enum vs union type a comparison

// const enum inlined at compile time
const enum HttpMethod {
  GET = "GET",
  POST = "POST",
  PUT = "PUT",
  DELETE = "DELETE",
}

function request(method: HttpMethod, url: string) {
  console.log(`${method} ${url}`);
}
request(HttpMethod.GET, "/api/users");
// Compiled output: request("GET", "/api/users") enum is gone

// Union literal often simpler, no special runtime behavior
type HttpMethodAlt = "GET" | "POST" | "PUT" | "DELETE";

function requestAlt(method: HttpMethodAlt, url: string) {
  console.log(`${method} ${url}`);
}
requestAlt("GET", "/api/users");
// requestAlt("PATCH", "/x");  // Error!

Common Mistakes

Using numeric enums in APIs or databases the numbers are meaningless outside TypeScript; use string enums or literal types for persisted/transmitted values
Forgetting that `const enum` cannot be used from external modules (it is inlined; there's no runtime object) use regular enums for library exports
Mixing enum values with raw numbers `if (status === 1)` instead of `if (status === Status.Active)` defeats the purpose

Note

For most projects, prefer union literal types (`type Status = "open" | "closed"`) over enums. They're simpler, work naturally with JSON serialisation, and don't require learning TypeScript-specific syntax. Use enums when you specifically need reverse mapping or are working in a codebase that already uses them.

Enums

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

What value does `Direction.North` have in `enum Direction { North, South, East, West }`?